It has been shown in [P. Ekstrand, “Bandwidth extension of audio signals by spectral band replication”, Proc. 1st IEEE Benelux Workshop on Model based Processing and Coding of Audio (MPCA-2002), pp. 53-58, Leuven, Belgium, 2002], that a complex-exponential modulated filter bank is an excellent tool for spectral envelope adjustment of audio signals. One application of this feature is audio coding based on Spectral Band Replication (SBR). Other fruitful applications of a complex filter bank include frequency selective panning and spatialization for parametric stereo, see [E. Schuijers, J. Breebart, H. Purnhagen, J. Engdegård: “Low complexity parametric stereo coding”, Proc. 116th AES convention, 2004, paper 6073] and parametric multichannel coding, see [J. Herre et al.: “The reference model architecture for MPEG spatial audio coding”, Proc. 118th AES convention, 2005, paper 6447]. In those applications the frequency resolution of the complex filter bank is further enhanced at low frequencies by means of sub-subband filtering. The combined hybrid filter bank hereby achieves a frequency resolution that enables the processing of spatial cues at a spectral resolution which closely follows the spectral resolution of the binaural auditory system. The additional filtering introduces no aliasing in itself, even if modifications are applied, so the quality of the hybrid filter bank is determined by the aliasing properties of the first filter bank.
If restraints on computational complexity prevent the usage of a complex exponential modulated filter bank, and only allows for a cosine modulated (real-valued) implementation, severe aliasing is encountered when the filter bank is used for spectral envelope adjustment. As shown in [0. Shamida et al.: “A low power SBR algorithm for the MPEG-4 audio standard and its DSP implementation”, Proc. 116th AES convention, 2004, paper 6048] adaptive subband gain grouping (or gain locking) can alleviate the aliasing to some extent. However, this method works best when only high frequency components of the signal have to be modified. For panning purposes in parametric multichannel coding, the amount of gain locking necessary to render the aliasing at lower frequencies inaudible will strongly reduce the frequency selectivity of the filter bank tool and will in practice render the additional frequency selectivity of a hybrid filter bank unreachable. The result is a rather narrow sound impression and problems with correct sound source placement. A much better compromise between quality and complexity would be obtained if the complex signal processing could be kept only for the perceptually more important lower frequencies.